Existance of Barrow Wights is connected with necromancy; from Morgoth's Ring of the History of Middle-earth series:
But it would seem that in these after-days more and more of the Elves,
be they of the Eldalie in origin or be they of other kinds, who linger
in Middle-earth now refuse the summons of Mandos, and wander houseless
in the world, unwilling to leave it and unable to inhabit it, haunting
trees or springs or hidden places that once they knew. Not all of
these are kindly or unstained by the Shadow. Indeed the refusal of the
summons (of Namo to Mandos) is in itself a sign of taint.
It is therefore a foolish and perilous thing, besides being a wrong
deed forbidden justly by the appointed Rulers of Arda, if the Living
seek to commune with the Unbodied, though the houseless may desire it,
especially the most unworthy among them. For the Unbodied, wandering
in the world, are those who at the least have refused the door of life
and remain in regret and self-pity. Some are filled with bitterness,
grievance, and envy. Some were enslaved by the Dark Lord and do his
work still, though he himself is gone. They will not speak truth or
wisdom. To call on them is folly. To attempt to master them and to
make them servants of one’s own will is wickedness. Such practices are
of Morgoth; and the necromancers are of the host of Sauron his
servant.
Some say that the Houseless desire bodies, though they are not willing
to seek them lawfully by submission to the judgement of Mandos. The
wicked among them will take bodies, if they can, unlawfully. The peril
of communing with them is, therefore, not only the peril of being
deluded by fantasies or lies: there is peril also of destruction. For
one of the hungry Houseless, if it is admitted to the friendship of
the Living, may seek to eject the fea from its body; and in the
contest for mastery the body may be gravely injured, even if it be not
wrested from its righful habitant. Or the Houseless may plead for
shelter, and if it is admitted, then it will seek to enslave its host
and use both his will and his body for its own purposes. It is said
that Sauron did these things, and taught his followers how to achieve
them."
Such elvish wights could be quite large in numbers (Avari elves were numerous, many could fall under Dark Power, of Morgoth or Sauron). And of course there were always those who practiced sorcery (Black Numenoreans, other peoples), many of them, ,,host of Sauron" implies large numbers.